Cellular communication systems are currently being developed and improved for machine type communication (MTC), which is a form of communication characterized by lower demands on data rates than for example mobile broadband, but with higher requirements on e.g. low cost device design, better coverage, and ability to operate for years on batteries without charging or replacing the batteries.
Currently, the Third Generation Partnership Project (3GPP) is standardizing Enhanced Machine-Type Communications (eMTC) (See e.g., 3GPP Tdoc RP-150492) as well as Narrowband Internet of Things (NB-IoT) [RP-152284] as part of LTE Release 13 for satisfying all the requirements put forward by MTC type applications, while maintaining backward compatibility with current LTE radio access technology.
eMTC
The eMTC features discussed in 3GPP Tdocs RP-152024 and R1-157926 include a low-complexity user equipment (UE) category called UE category M1 (or Cat-M1 for short) and coverage enhancement (CE) techniques (CE modes A and B) that can be used together with UE category M1 or any other LTE UE category.
All eMTC features (both Cat-M1 and CE modes A and B) operate using a reduced maximum channel bandwidth compared to normal LTE. The maximum channel bandwidth in eMTC is 1.4 MHz whereas it is up to 20 MHz in normal LTE. The eMTC UEs are still able to operate within the larger LTE system bandwidth without problem. The main difference compared to normal LTE UEs is that the eMTCs can only be scheduled with 6 physical resource blocks (PRBs) of 180 kHz each at a time.
The restricted channel bandwidth of at most 6 PRBs also means that system information (known as SIB1-BR or SIB1bis) transmissions that are broadcasted to eMTC UEs need to be restricted to a maximum of 6 PRBs.
To achieve good frequency diversity even though the channel bandwidth is relatively small, frequency hopping across the LTE system bandwidth is applied to the SIB1bis transmissions. The number of frequency positions that the hopping occurs between depends on the LTE system bandwidth. For the largest system bandwidth of 20 MHz, the SIB1bis transmission hops between 4 different narrowbands (NBs) of 6 PRBs each.
3GPP RAN WG1 meeting #83 agreed on the following frequency hopping for SIB1bis:                S is a set of valid DL narrowbands for SIB1bis, the narrowband is indexed in the order of increasing value                    S={s0, s1, s2, . . . , sK-1}, K=number of valid DL narrowbands for SIB1bis                        For system BW less than 12 RBs                    SIB1bis is transmitted in narrowband sj where j=PCID mod K                        For system BW between 12-50 RBs                    1st NB is sj where j=PCID mod K            2nd NB is (sj+floor(K/2)) mod K            Starting at SFN mod 8=0, SIB1bis transmission cycles through {1st NB, 2nd NB}                        For system BW between 51-110 RBs                    1st NB is sj where j=PCID mod K            2nd NB is (sj+floor(K/4)) mod K            3rd NB is (sj+2*floor(K/4)) mod K            4th NB is (sj+3*floor(K/4)) mod K            Starting at SFN mod 8=0, SIB1bis transmission cycles through {1st NB, 2nd NB, 3rd NB, 4th NB}                        
FIG. 1 illustrates how an LTE system bandwidth of 10 MHz (50 PRBs) is divided into 8 NBs of 6 PRBs each. The remaining 50−8*6=2 PRBs can be found at the band edges (and in case of LTE system bandwidths consisting of an odd number of PRBs, the one PRB at the center of the LTE system bandwidth is also unused). The SIB1bis is mapped to the NBs that do not overlap with the central 72 subcarriers where the PSS/SSS/PBCH transmissions may occur—this corresponds to the central 6 PRBs in the 10-MHz example in FIG. 1 (one PRB consists of 12 subcarriers). This means that in this 10-MHz example, the SIB1bis transmission can occur in 6 of the 8 NBs, as marked with “narrowband index SIB1bis” in the 3rd column of FIG. 1. For a cell of 10 MHz (=50 PRBs) system bandwidth, frequency hopping of SIB1bis occurs between 2 narrowbands. Thus for a given cell of a particular PCID value, frequency hopping of SIB1bis occurs between 2 narrowbands, which are chosen from the 6 NBs using the equation above.
Furthermore, to achieve inter-cell interference randomization between different cells in the network, these up to 4 frequency positions depend on the physical cell identity (PCID) of the cell, which means that the frequency positions for SIB1bis transmission can be different in different cells.
The UE knows the PCID of the cell from the synchronization signals (PSS and SSS) in the cell and it knows the LTE system bandwidth and system frame number (SFN) from the master information block (MIB) conveyed on the physical broadcast channel (PBCH) in the cell.
The eMTC UE furthermore receives information about the SIB1bis scheduling in the MIB. 5 bits in the MIB are used to encode information about the transmission block size (TBS) of the SIB1bis and the number of repetitions (R) within an 80-ms scheduling period as described in Table 1. As can be seen from the table, only 19 of the 32 possible values of these 5 bits are currently used. This is described, for instance, in 3GPP Tdoc R1-157926, section 7.1.7.2.7.
TABLE 1SIB1bis scheduling information in MIBValueTBSR0No SIB1bis transmission in the cell1TBS142TBS183TBS1164TBS245TBS286TBS2167TBS348TBS389TBS31610TBS4411TBS4812TBS41613TBS5414TBS5815TBS51616TBS6417TBS6818TBS61619-31Reserved
Furthermore, 3GPP RAN WG1 meeting #83 agreed on the following regarding paging-related transmissions:                How to indicate set of M-PDCCH narrowband(s) for paging                    implicit mapping                            1st narrowband=PCID mod N_NB_paging                Other narrowbands are in consecutive order from 1st narrowband with wrap-around                                                When more than one narrowband is configured for M-PDCCH for paging                    The narrowband is up to RAN2                        
The specifications are not finalized yet but what this means is that also paging-related transmissions may occur in 6-PRB narrowband which is determined by the PCID.
NB-IoT operation modes
For NB-IoT, three different operation modes are defined, i.e., standalone, guard-band, and in-band. In standalone mode, the NB-IoT system is operated in a dedicated frequency band, e.g., refarming one or more GSM channels and use it as NB-IoT PRB. For in-band operation, the NB-IoT system can be placed inside the frequency bands used by the current LTE system, while in the guard-band mode, the NB-IoT system can be placed in the guard band used by the current LTE system. The NB-IoT has a system bandwidth of 180 kHz, i.e. substantially smaller than the LTE system bandwidth which is in the range from 1.4 MHz to 20 MHz.
The channel raster of the NB-IoT systems is on a frequency grid of 100 kHz. That is, the NB-IoT devices try to search for the NB-IoT carriers in a step size of 100 kHz. For the standalone deployment where one or more GSM channels are refarmed, this is not a problem since GSM channel bandwidth is 200 kHz and the center frequency is in the 100 kHz grid. But for the in-band and guard-band operations, due to the presence of the DC-carrier and the fact the center of the PRB is in between two sub-carriers, there is no PRB that falls directly on the cell search grid used in LTE in-band operation. The frequency offset to the 100 kHz grid is a minimum of ±2.5 kHz and ±7.5 kHz for even and odd number of PRBs in the LTE system bandwidth, respectively. In an NB-IoT receivers, algorithms can be designed such that the ±2.5 kHz or ±7.5 kHz offset can be handled by the device during the cell search process without degrading the synchronization performance considerably. However, larger offset values are more problematic, and the receiver may not be able to handle larger offsets. Therefore, the NB-IoT carriers are constrained to certain positions for the in-band and guard-band operations.
After the initial cell search phase, the NB-IoT devices need the knowledge of the operation mode to know the reference signal placements, to avoid collisions with legacy systems, and to decode the received data correctly. For in-band and guard-band operations, additional carrier frequency offset caused by channel raster step size, can cause potential performance degradations for NB-IoT devices to decode the Master Information Block (MIB) carried by the Narrow Band Physical Broadcast CHannel (NB-PBCH), especially the devices in really low SNR. Receiver algorithms can help to reduce the degradation, e.g., the receiver can try to hypothesizing and compensate different offset values before the NB-PBCH decoding.
NB-IoT Operation in-Band in LTE Carrier
FIG. 2 shows the center frequency offsets for the LTE PRBs closest to the center frequency of the LTE carrier.
NB-IoT Operation in LTE Guard Band
FIG. 3 illustrates an adjacent LTE PRB for guard band operation in 10 MHz LTE system bandwidth.
In guard band operation, to maintain orthogonality to the LTE carrier and minimize the ACLR to adjacent LTE carrier, placing the NB-IoT PRB on the 15 kHz sub-carrier grid using the 12 “next” subcarriers adjacent to the LTE carrier is desired as shown in FIG. 3 for the 10 MHz LTE system bandwidth.
For the guard-band operation, for an LTE system with 10 or 20 MHz system bandwidth, it is possible to find NB-IoT carrier frequency that is ±2.5 kHz off the 100 kHz frequency raster. For other LTE system bandwidth, the offset to the 100 kHz raster is 52.5 kHz. Therefore, to get within the same ±7.5 kHz to the 100 kHz grid, 3 guard subcarriers are needed. One guard carrier is 15 kHz wide and placed in the same FFT grid at the legacy LTE system that gives orthogonality to the legacy LTE PRB. However, there are no other solutions to put the NB-IoT carriers on the exact 100 kHz raster grids on the LTE guard-band without losing orthogonality to the legacy LTE system.
Table 2 shows the center frequency offset for this adjacent PRB. Only the higher frequency guard band is shown, but the offset is the same to the adjacent PRB in the lower guard band. The 1.4 MHz system bandwidth has been excluded since guard band operation is not seen as feasible.
TABLE 2Center frequency offset of the guard bandPRB for different LTE system bandwidthsGuard band PRBGuard bandGuard subcarrierscenter frequencySystemPRBs inPRB centerOffset toneeded to beoffset whenbandwidthsystemfrequency100 kHzwith ±7.5 kHzincluding guard[MHz]bandwidthoffset[kHz]gridof 100 kHzsubcarriers [kHz]3151447.552.531492.55252347.552.532392.510504597.52.504597.515756847.552.536892.5201009097.52.509097.5